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cxl: docs/allocation/reclaim

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Document a bit about how reclaim interacts with various CXL
configurations.

Signed-off-by: Gregory Price <gourry@gourry.net>
Link: https://patch.msgid.link/20250512162134.3596150-16-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
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Gregory Price 2025-05-12 12:21:32 -04:00 committed by Dave Jiang
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Documentation/driver-api/cxl/allocation/reclaim.rst Normal file
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.. SPDX-License-Identifier: GPL-2.0
=======
Reclaim
=======
Another way CXL memory can be utilized *indirectly* is via the reclaim system
in :code:`mm/vmscan.c`. Reclaim is engaged when memory capacity on the system
becomes pressured based on global and cgroup-local `watermark` settings.
In this section we won't discuss the `watermark` configurations, just how CXL
memory can be consumed by various pieces of reclaim system.
Demotion
========
By default, the reclaim system will prefer swap (or zswap) when reclaiming
memory. Enabling :code:`kernel/mm/numa/demotion_enabled` will cause vmscan
to opportunistically prefer distant NUMA nodes to swap or zswap, if capacity
is available.
Demotion engages the :code:`mm/memory_tier.c` component to determine the
next demotion node. The next demotion node is based on the :code:`HMAT`
or :code:`CDAT` performance data.
cpusets.mems_allowed quirk
--------------------------
In Linux v6.15 and below, demotion does not respect :code:`cpusets.mems_allowed`
when migrating pages. As a result, if demotion is enabled, vmscan cannot
guarantee isolation of a container's memory from nodes not set in mems_allowed.
In Linux v6.XX and up, demotion does attempt to respect
:code:`cpusets.mems_allowed`; however, certain classes of shared memory
originally instantiated by another cgroup (such as common libraries - e.g.
libc) may still be demoted. As a result, the mems_allowed interface still
cannot provide perfect isolation from the remote nodes.
ZSwap and Node Preference
=========================
In Linux v6.15 and below, ZSwap allocates memory from the local node of the
processor for the new pages being compressed. Since pages being compressed
are typically cold, the result is a cold page becomes promoted - only to
be later demoted as it ages off the LRU.
In Linux v6.XX, ZSwap tries to prefer the node of the page being compressed
as the allocation target for the compression page. This helps prevent
thrashing.
Demotion with ZSwap
===================
When enabling both Demotion and ZSwap, you create a situation where ZSwap
will prefer the slowest form of CXL memory by default until that tier of
memory is exhausted.

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Documentation/driver-api/cxl/index.rst
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@ -46,5 +46,6 @@ that have impacts on each other. The docs here break up configurations steps.
allocation/dax
allocation/page-allocator
allocation/reclaim
.. only:: subproject and html